Which of the following is endothermic process :

Options

A. `CI(g)+e^(-)rarr CI^(-)(g)`
B. `S^(-)(g)+e^(-)rarrS^(2-)(g)`
C. `Na^(+)(g)+e^(-)rarr Na(g)`
D. `O^(2-)(g)rarr O^(-)+e^(-)`

Answer

To determine which of the given processes is endothermic, we need to analyze each process based on the change in enthalpy (ΔH). An endothermic process is characterized by a positive ΔH, meaning that heat is absorbed from the surroundings.

### Step-by-Step Solution:

1. **Identify the Processes:**
- Process A: Isolated gaseous chlorine atom (Cl) gaining an electron to become Cl⁻.
- Process B: Uninegative sulfur ion (S⁻) gaining an electron to become S²⁻.
- Process C: Sodium ion (Na⁺) gaining an electron to become neutral sodium (Na).
- Process D: O²⁻ gaining an electron to become O⁻.

2. **Analyze Process A:**
- Cl + e⁻ → Cl⁻
- This process releases energy, indicating it is exothermic (ΔH < 0).

3. **Analyze Process B:**
- S⁻ + e⁻ → S²⁻
- The uninegative sulfur ion is unstable, and adding an electron requires energy input. Thus, ΔH > 0, making this process endothermic.

4. **Analyze Process C:**
- Na⁺ + e⁻ → Na
- This is the reverse of the ionization process, which is exothermic. Therefore, ΔH < 0, indicating it is exothermic.

5. **Analyze Process D:**
- O²⁻ + e⁻ → O⁻
- This process converts a less stable species to a more stable one, releasing energy. Hence, ΔH < 0, making it exothermic.

6. **Conclusion:**
- Among the processes analyzed, only Process B (S⁻ gaining an electron to become S²⁻) has a positive ΔH, indicating that it is the endothermic process.

### Final Answer:
The endothermic process is **Process B: Uninegative sulfur ion (S⁻) gaining an electron to become S²⁻.**

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